GSM is a uniform, intelligent digital mobile telephone system which is not geographically confined to a single country. A subscriber may use any subscriber unit (Mobile Station--MS) by controlling it with the aid of a subscriber identity module (SIM), which may be an active card or a plug-in unit inserted in the subscriber unit and producing the subscriber's identity (IMSI) which is allocated to a directory number (Mobile Station International ISDN number--MSISDN) . Information on IMSI and MSISDN is stored, together with other information relating to the subscriber, in a home database (Home Location Register--HLR) with the operator of the network including the subscriber. The system comprises visitor databases (Visitor Location Register--VLR) and switches (Mobile Services Switching Centre--MSC) Information on an activated subscriber unit MS is temporarily stored in the VLR pertaining to the area of location of the subscriber unit MS.
Since the above concepts, as well as the design and operation of the system as a whole, are subjected to extensive standardisation, no detailed description seems required here.
Systems of the above type are widely used for service calls, but also to some extent for private calls. Distributing the costs for service and private calls usually cause problems or involves extra work.
One and the same subscription can also be used by different people, e.g. within one and the same company. Also in this case, distributing the costs among the different people concerned may cause problems or involve extra work.